1. Field of the Invention
The present invention relates to an internal combustion engine control apparatus in which maximum power and maximum efficiency can be obtained in spite of scatter in performance of engines.
2. Prior Art
Heretofore, there has been used an apparatus for controlling a fuel injection valve and an ignition device by calculating a proper fuel supply quantity and ignition timing on the basis of the relationship between a suction air quantity or suction pipe pressure and an engine speed (rpm).
Further, a control apparatus designed to perform higher-precision control by detecting the combustion pressure of the engine and adjusting the pressure to a predetermined value has been disclosed in Japanese Pat. Unexamination Publication No. 62-85148.
In this control apparatus, the combustion condition is detected by the output of a cylinder internal pressure (combustion pressure) sensor provided in every cylinder so that the controlling of combustion injection timing, EGR (exhaust gas recycle) valves and the like are carried out to fit the condition in a predetermined pattern.
In the aforementioned prior art apparatus, control is carried out to fit the combustion pressure to a combustion pattern determined in advance by a standard engine. In the case where a large number of engines are mass-produced, there arises considerable scatter. Accordingly, individual engines require individually different combustion patterns. For this reason, it cannot be absolutely said that precision in controlling is improved by controlling the combustion pressure by use of a uniform standard pattern. On the contrary, the performance of the engine may be rather lowered by such control.
Further, in the prior art apparatus, the fuel injection timing, the EGR rate and the like are controlled as operation parameters for controlling the combustion pressure. However, the most effective parameters for the output performance of the engine are the combustion injection quantity and ignition timing optimum thereto.
In general, the freely controllable range of the fuel injection quantity is limited for the purpose of suppressing the component concentration of the exhaust gas to a low level. Accordingly, it is necessary to control the fuel injection quantity and the ignition timing comprehensively to reconcile the components of the exhaust gas and the power performance of the engine.
Further, in a gasoline engine, in order to clean up exhaust gas and improve the output power of the engine, it is necessary to properly control the air-fuel ratio and ignition timing in accordance with the operating condition of the engine. Therefore, a method using a micro-computer to control the air-fuel ratio and ignition timing has been widely used in the field of car gasoline engine.
For example, the air-fuel ratio control based on the quantity of fuel injection is carried out in such a manner as follows. The quantity of suction air (Q.sub.a) in the engine is detected by an air-flow sensor provided in an air-intake passage. The engine speed or the number of engine revolutions per unit time (N.sub.e) is obtained from the output of a rotation sensor provided on a crankshaft or the like. The quantity of air per engine revolution (Q.sub.a /N.sub.e) is calculated and, accordingly, the quantity of basic fuel injection is calculated based on the quantity of air (Q.sub.a /N.sub.e). The quantity of basic fuel injection is used for the purpose of obtaining a target air-fuel ratio at every predetermined operation point. Then, a correction is carried out in accordance with the output of a water sensor or the like provided to detect the temperature of engine cooling water to thereby finally decide the quantity of fuel injection. On the basis of an injection pulse signal having a pulse width corresponding to the thus decided fuel injection quantity, an injector is actuated to open its valve in synchronism with the rotation of the engine to inject fuel into, the engine. Further, in a low and partial load range, an air-fuel ratio sensor in which the output thereof rapidly changes in the vicinity of the theoretical air-fuel ratio is used to thereby judge whether the actual air-fuel ratio is on a rich side or on a lean side. A feedback correction based on the judgment is applied to the quantity of fuel injection so that the air-fuel ratio of the engine is controlled so as to be converged into the theoretical air-fuel ratio. By controlling the air-fuel ratio to be always the theoretical air-fuel ratio, cleaning of exhaust gas can be carried out with high efficiency by used of ternary catalystic method.
On the other hand, the control of ignition timing is carried out in such a manner as follows. In general, an ignition timing advance predetermined in the form of a map corresponding to the air quantity (Q.sub.a /N.sub.e) and the engine speed (N.sub.e) is read. The current conduction of an ignition coil is controlled by an ignition signal based on the thus read-out ignition timing advance.
In general, the ignition timing advance is determined so as to aim at MBT (minimum ignition timing advance required for producing maximum engine torque). Because MBT varies widely according to several factors, such as scatter in engine temperature and air-fuel ration, dimensional error in the combustion chamber, temperature and humidity of suction air and the like, it is difficult to obtain optimum ignition timing continuously by such a simple "open" control method. There exists a problem in that knocking trouble may occur or reduction of torque may occur. Therefore, such an improvement has been proposed as described in Japanese Pat. Unexamination Publication No. 62-82273. The improvement is constructed so that the ignition timing feedback control is carried out on the basis of the measured value of cylinder internal pressure to maximize the engine torque. According to the cylinder internal pressure feedback control method, for example, a rotation sensor is provided to generate a pulse for every degree (1.degree. C.) of crank angle. The output value (P.sub.0) of the cylinder internal pressure sensor measured for every pulse generation is read successively, so that mean effective pressure (P.sub.1 ) represented by the following equation is calculated from cylinder volume (V) and piston displacement (V.sub.n) corresponding to the currently obtained crank angle. ##EQU1## Consequently, the ignition timing feedback control is carried out to maximize P.sub.1.
As described above, the conventional cylinder internal pressure feedback controlling method has an attempt to obtain maximum torque by correcting ignition timing to maximize mean effective pressure (P.sub.1). However, the air-fuel ratio feedback control is stopped in a high load range, so that "open" control is carried out in the high load range. Accordingly, air-fuel ratio error caused by scatter in characteristics of sensors, injectors and fuels is produced as reduction of torque. Accordingly, it is impossible to draw out the best in the torque of the engine even if cylinder internal pressure feedback control is carried out by correcting ignition timing.